An electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles suspended in a colored dielectric solvent. This general type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between the electrodes. At least one of the electrodes, typically on the viewing side, is transparent.
When a voltage difference is imposed between the two electrodes, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages or pulsing time.
EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD (M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148-1152 (1979)) and the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026).
An improved EPD technology was recently disclosed in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO 01/67170 published on Sep. 13, 2001), U.S. Ser. No. 09/759,212, filed on Jan. 11, 2001 (corresponding to WO 02/56097), U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (corresponding to WO 02/01281 published on Jan. 3, 2002) and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001 (corresponding to WO 02/65215), all of which are incorporated herein by reference. The improved EPD comprises isolated cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged particles dispersed in a dielectric solvent, preferably a halogenated solvent, particularly a perfluorinated solvent. The filled cells are individually sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from a group consisting of thermoplastics, thermosets and precursors thereof. The microcup based display cells may be manufactured by microembossing or photolithography.
To improve the image quality and longevity of the electrophoretic displays, the surface properties of the microcup must be optimized to reduce the undesirable irreversible particle deposition or network formation. To achieve optimum switching performance, the dielectric properties and/or conductivity of the microcup material and its interaction with the electrophoretic fluid also need to be optimized. However, to facilitate mold release during microembossing, a releasing agent such as a silicone oil, a perfluoro compound or vinyl or acrylate derivatives thereof is often applied to the mold or incorporated in the embossing resin composition. The requirement of a good mold release property unfortunately tends to result in a low surface tension and hydrophobic microcup surface of a low dielectric constant which severely narrows the processing and formulation windows for acceptable display performance.